Planar lightwave circuits such as waveguide gratings and optical switches control the routing of optical signals. In order to accomplish this control or routing, input and output optical fibers are connected to the planar lightwave circuit (PLC). A convenient method to align and manage more than one fiber is to use fiber assemblies.
FIG. 1A shows the example of an optical switch 100 with four fiber assemblies 110A, 110B, 110C, and 110D for four sets of optical fibers 112. The four fiber assemblies 110A, 110B, 110C, and 110D are connected to an optical plate 120 that forms a PLC. In this example, fiber assemblies 110A and 110D include optical fibers 112 that input optical signals to optical plate 120 and fiber assemblies 110B and 110C include optical fibers 112 that receive optical signals output from optical plate 120.
In general, optical plate 120 can be made of any material in which optical waveguides can be created. These materials generally have low optical loss for the target wavelengths and a refractive index profile can be created perpendicular to the propagation direction so as to guide the light. In the example of FIG. 1A, optical plate 120 is made of an optical material such as fused silica that is selectively doped with impurities to form optical waveguides, but waveguides can be formed in other structures such as in semiconductor lasers and Lithium Niobate (LiNbO3) modulators.
Optical plate 120 includes two example sets of optical waveguides 122 and 124. Optical waveguides 122 are aligned with optical fibers 112 in fiber assemblies 110A and 10C, and optical waveguides 124 are aligned with fibers 112 in fiber assemblies 110B and 110C. Switching sites 126 that select the paths of the optical signals are at the intersections of optical waveguides 122 and 124.
In operation, switching sites 126 can be individually turned on or off so that an optical signal input to an optical waveguide 122 or 124 either reflects at one of the switching sites 126 along the waveguide 122 or 124 into another optical waveguide 124 or 122 or passes through every switching site 126 along the optical waveguide 122 or 124. In one specific implementation, each switching site 126 includes a trench in optical plate 120 that is either filled with a liquid to make the switching site 126 transparent or filled with a gas bubble to make the switching site 126 reflective. An integrated circuit (not shown) underlying optical plate 120 can selectively heat the liquid in a particular switching site 126 to create the gas bubble that turns on that switching site 126 and makes that switching site 126 reflective.
Optical switch 100 can route an optical signal from an optical fiber 112 in fiber assembly 110A, for example, into any of the optical fibers 112 in fiber assembly 1110B by making the appropriate switching sites 126 reflective. Alternatively, if none of the switching sites 126 along the optical waveguide 122 are reflective, the optical signal from the optical fiber 112 in fiber assembly 110A passes through optical plate 120 to an optical fiber 112 in the opposite fiber assembly 110C.
Proper operation of optical switch 100 requires that the spacing of optical fibers 112 on each fiber assembly 110A, 110B, 110C, or 110D match the spacing of input/output areas for the corresponding optical waveguides 122 or 124. Additionally, the optical fibers 112 must be precisely aligned with optical waveguides 122 or 124 and with optical fibers 112 in other fiber assemblies to achieve maximum performance. Fabricating and aligning fiber assemblies with the required precision can present difficulties because waveguides 122 and 124 have typical dimensions of about 10 xcexcm or less and a standard optical fiber 112 has a diameter of 125 xcexcm and a core 10 xcexcm in diameter. The cores of the optical fibers 112 carry the optical signals and must be aligned for transfer of optical signals to or from the corresponding waveguide. Accordingly, for maximum performance the spacing and alignment of the optical fibers 112 typically must be accurate to within a few tenths of a micron.
FIG. 1B shows a cross-sectional view of a fiber assembly 110. Fiber assembly 110 includes a substrate 115 having v-grooves 116 in which optical fibers 112 reside. Substrate 115 is typically made of the same material as the optical plate (e.g., fused silica) to provide a matching coefficient of thermal expansion (CTE), but other materials such as silicon can also be used.
Precision machining of substrate 115 can produce v-grooves 116 with consistent shape and spacing. Such machining can use, for example, step and repeat techniques that grind a v-groove 116 in substrate 115 then move substrate 115 the required distance for grinding the next v-groove 116 in substrate 115. Equipment including a precision stage that positions substrate 115 for grinding can achieve the required precision for the spacing of v-grooves 116. However, separate mechanical operations such as cutting an edge 118 of substrate 115 generally require remounting substrate 115 on different equipment, which introduces variations greater than the required alignment precision. Accordingly, the position of edge 118 of substrate 115 relative to v-grooves 116 may vary by xc2x125 xcexcm.
An exemplary process for aligning fiber assemblies 110A, 110B, 110C, and 110D with optical plate 120 as in FIG. 1A includes a coarse alignment process and a fine alignment process. The coarse alignment process aligns fiber assemblies 110A, 110B, 110C, and 110D and optical plate 120 with sufficient precision to provide some light flow through the required paths. A fine alignment process measures the intensity of output optical signals and adjusts the positions and orientations of assemblies 120 to maximize optical power flow through switch 100. Fine alignment can be computer controlled using known xe2x80x9chill climbingxe2x80x9d algorithms that find the optimal position and orientation for the fiber assemblies 110A, 110B, 110C, and 110D.
Coarse alignment of an assembly 110 and an optical plate 120 aligns the cores 114 of optical fibers 112 with respective optical waveguides 122 or 124 in optical plate 120 so that optical signals flow through optical switch 100. Coarse alignment initially relies on identifying and matching physical features of fiber assembly 110 and optical plate 120. However, cores 114, which are to be aligned, are indistinguishable from other portions of optical fibers 112, and the optical fibers 112, which have their protective sheathes removed for accurate assembly, are transparent and therefore difficult to identify using machine or human vision. Features such as v-grooves 116 or their edges are similarly difficult to identify, particularly when substrate 115 is transparent. Separate mechanically made features such as edges 118 of substrate 115, which may be easier to identify, are subject to variations much greater than those required in the coarse alignment.
The difficulties in identifying reliable reference features for coarse alignment typically means that the coarse alignment is conducted manually. Additionally, an alignment based solely on the apparent location of the features often fails to provide adequate optical power transmission for the fine alignment process. Accordingly, the coarse alignment must further include a search process that systematically shifts or reorients the fiber assemblies until achieving a configuration with sufficient optical power transmission for the fine alignment process. Such coarse alignment procedures can take an hour or more, while computer-controlled fine alignment can typically be completed in two to ten minutes. Accordingly, structures and techniques are sought that can reduce the time required for aligning fiber assemblies in optical switches or other PLCs.
In accordance with an aspect of the invention, both a fiber assembly and an optical plate containing a light circuit have fiducials for coarse alignment of the fiber assembly during fabrication of an optical device. In a fiber assembly including a substrate with machined grooves for optical fibers, a fiducial can be disposed in one of the grooves so that the accuracy of the reference position that the fiducial provides is approximately the same as the accuracy of the positions of the optical fibers. In one embodiment, the fiducial on the fiber assembly is an opaque fiber such as a carbon-coated optical fiber. The centroid of the opaque fiber marks the center of the groove containing the opaque fiber and indicates to the accuracy with which the grooves were formed the positions of other grooves and the optical fibers in the other grooves. As an alternative to the opaque fiber, any opaque or easily visible structure such as a wire or a hypodermic needle having a uniform diameter or thickness can be placed in a groove, or the groove can be otherwise filled with an opaque material.
Photolithographic processes can form optical waveguides, switching sites, and fiducials in the optical plate. Since photolithographic processes conventionally use alignment marks to align successive operations, such processes can provide the required positional accuracy for the fiducials formed on the optical plate even if formation of the fiducials is before or after the processes that form the optical waveguides and switching sites.
In accordance with a further aspect of the invention, photolithographic processes can form grooves in a substrate for a fiber assembly and form fiducials as regions of opaque material on the substrate. Unlike mechanical processes that generally do not use alignment marks for precise alignment of separate processes, the photolithographic processes can position the fiducials accurately relative to the grooves and thereby permit use of the fiducials for aligning the fiber assembly with an optical plate.
Machine vision, interferometer measurements, or other computer controllable processes using appropriate sensors can identify the positions and orientations of fiducials on fiber assemblies and on an optical plate during alignment of the assemblies. Using the appropriate sensors, the coordinates for all 6 degrees of freedom can be identified for both parts. Based on the identified positions and orientations, the computer-controlled alignment process moves the fiber assemblies relative to the optical plate to the coarsely aligned positions that reliably provide light flow through the device. This is much faster than the manual searching technique described above. A fine alignment process can then use xe2x80x9chill climbingxe2x80x9d techniques to position the fiber assemblies for maximum power output.
One specific embodiment of the invention is a process for making an optical device. The process includes fabricating a fiber assembly having a plurality of optical fibers and a first fiducial on a substrate and fabricating an optical plate having a second fiducial. The first fiducial can be an opaque object such as a carbon-coated fiber in a groove that is substantially identical to grooves containing the optical fibers. Alternatively, the first and/or second fiducials can be formed using photolithographic processes that provide the required precision for the positions of the first and second fiducials relative to optical fibers and optical waveguides, respectively.
With the fiber assembly and optical plate thus fabricated, the process further includes identifying locations for the first and second fiducials and moving the fiber assembly relative to the optical plate until the first and second fiducials reach a target relative position. The target relative position provides coarse alignment of the fiber assembly and the optical plate. The process can further include fine alignment that measures optical power flowing through fiber assembly and the optical plate and adjusts the relative position of the fiber assembly and optical plate to maximize the optical power.
Identifying the locations for the first and second fiducials can be done by applying computer vision to an image of the fiber assembly and the optical plate and then computing a relative movement of the fiber assembly and/or the optical plate required to reach the target relative positions. Alternatively, a measuring device such as an interferometer can measure distances to the first and second fiducials and fiducial edges for angular information. A relative movement of the fiber assembly and the optical plate required to reach the target relative positions can be computed from the measurements.
Another embodiment of the invention is an optical device such as a fiber assembly or an optical switch. The device generally includes a substrate having grooves formed in a surface. Optical fibers are in a set of the grooves on the surface of the substrate, and an opaque fiducial is in a groove that does not contain an optical fiber for light guiding. The fiducial can be an opaque cylindrical object such as a carbon-coated fiber. For an embodiment of an optical switch, the device further includes an optical plate to which the substrate is attached. The optical plate contains a light circuit including optical waveguides that are respectively aligned with the optical fibers. In contrast, the opaque fiducial is aligned with a portion of the optical plate that is not a functional optical waveguide for optical signals.